Cell cycle checkpoints are vital mechanisms for the maintenance of genomic stability. The primary function of the damage and replication checkpoints is to attenuate the cell cycle progression at distinct stages such as G1/S, S, and G2/M, allowing the repair of DNA lesions and/or recovery of disrupted replication structure. This course of action prevents conversion of aberrant DNA structures into heritable mutations. Deficiencies in the checkpoint pathways will inevitably result in accumulation of mutations and chromosomal abnormalities, and eventually lead to tumorigenesis. As a biochemical pathway, the damage and replication checkpoint can be dissected into three functional steps: damage sensing/signal initiation, signal transduction/processing, and targeting of the cell cycle machinery. Investigations during the past decade have identified most of the key components of the damage and replications checkpoints in both yeast and humans. The molecular mechanisms of checkpoint signal transduction and cell cycle targeting have also been elucidated to a great extent. The damage sensing and signal initiation step, however, remains largely unclear. Our proposed studies are aimed at delineating the mechanisms of checkpoint signal initiation. Our focus will be on two gene products, hRad17 and hRad1, both of which are crucial factors during the initiation of checkpoint signals. Recent studies from our group and others have shown that the hRad17 protein is a target of the Atm/Atr kinase and is likely a recruiting factor for the loading of the hRad1/hRad9/hHus1 trimer to the site of DNA damage during activation of the checkpoint. We have constructed a somatic conditional knockout model for hRAD17 in human cell and will create a human cellular model for the hRAD1 gene. Establishing these genetic models give us unique tools to study the function of hRAD1 and hRAD17 in cell proliferation, DNA damage-induced cell cycle arrest, and maintenance of chromosomal stability. Our results are expected to further elucidate the mechanisms of the DNA damage and replication checkpoint, and the molecular basis of genomic instability at large.